Variable capacity wobble plate compressor

ABSTRACT

A controlled, variable displacement, wobble plate compressor includes a pivotable wobble plate driven through an improved drive and support linkage including a universal joint and having its pivot point offset from the axis of the drive shaft so that at the &#39;&#39;&#39;&#39;no stroke&#39;&#39;&#39;&#39; position the pistons are all disposed at their top-dead-center positions. The capacity of the compressor is controlled by an improved system for varying the position of the wobble plate.

United States Patent [191 Roberts et al.

[ 1 Jan. 21, 1975 VARIABLE CAPACITY WOBBLE PLATE COMPRESSOR Inventors:Richard W. Roberts, Lombard;

Ralph D. Salle, Lake Zurich, both of 111.

Borg-Warner Corporation, Chicago, Ill.

Filed: Apr. 4, 1973 Appl. No.: 347,759

Assignee:

US. Cl 417/53, 417/212, 417/270 Int. Cl F04b 49/00 Field of Search417/269, 270, 222, 53;

References Cited UNITED STATES PATENTS 10/1912 Macomber 92/12.]

l/1932 Rayburn 92/122 2/1941 Wahlmark 92/121 5/1946 Joy 417 2222,573,863 11/1951 Mitchell 417/270 2,929,551 3/1960 Heidorn 417/2702,964,234 12/1960 74/60 3,010,403 1l/l96l 417/269 3,705,493 12/197260/444 3,712,759 l/l973 Olson 417/269 Primary Examiner-William L. FreehAssistant ExaminerG. P. LaPointe Attorney, Agent, or Firm-Thomas B.Hunter [57] ABSTRACT A controlled, variable displacement, wobble platecompressor includes a pivotable wobble plate driven through an improveddrive and support linkage including a universal joint and having itspivot point offset from the axis of the drive shaft so that at the nostroke" position the pistons are all disposed at their top-dead-centerpositions. The capacity of the compressor is controlled by an improvedsystem for varying the position of the wobble plate.

7 Claims, 6 Drawing Figures PATENTEB JANZI I915 SHEET 2 BF 4 VARIABLECAPACITY WOBBLE PLATE COMPRESSOR BACKGROUND AND SUMMARY OF THE INVENTIONThis invention relates generally to rotary wobble plate compressors andmore particularly to an improved wobble plate construction wherein thewobble plate is pivoted from a point radially spaced or offset from theaxis of the drive shaft so that the no-stroke position, i.e. thatposition which results in negligible piston movement, corresponds to thetop-dead-center position of the pistons.

The advantages of wobble plate type compressors are well recognized inthe art. Such compressors (or fluid motors) utilize a plate, theposition of which is movable out of a plane normal to the direction ofpiston travel to permit varying of the stroke and thereby thedisplacement or capacity of such units. Essentially, such compressorsmay be considered rotary devices in that a substantial portion of themass is undergoing a rotary motion, thereby contributing to thesmoothness and quietness of operation. On the other hand, the pistonsreciprocate and thereby have the advantage of reciprocating pistondevices with their convenient adaption for capacity control andefficient sealing. One main difference between the wobble plate typecompressor and the conventional reciprocating compressor is that thepistons travel in the same direction as the axis of the rotating driveelement, as distinguished from the radially reciprocating motion ofpistons in a conventional reciprocating piston device. As is well knownin the art, with three of more uniformly spaced, axially reciprocatingpiston masses, the resulting unbalance is a rotating couple. This couplecan be balanced by suitably disposed masses on the rotating shaft, andis usually accomplished in devices of this type by properly sizing theinclined rotating element. These features are responsible for a muchsmoother, more vibrationless and quieter apparatus, while combining theefficiency of the reciprocating pistons as a gas compressing element.

Heretofore, most wobble plates have been constructed so that they pivotaround a point lying along the axis of the drive shaft. While this issatisfactory for hydraulic pumps or motors, it creates a problem withrespect to compressing a fluid such as conventional halocarbonrefrigerants. In the fully unloaded or nostroke position, the pistonsare disposed approximately half way between their top-dead-center andbottomdead-center positions. With this type of mechanism, when reducingthe stroke from the full stroke condition, the relative clearance volumeincreases very rapidly with a decrease in stroke. At high pressureratios, this drastically reduces the pumping capability of thecompressor and it will cease to deliver any flow at all long before thezero stroke position is reached.

In the present invention, the wobble plate is adapted to be pivoted froma point which will result in minimal clearance volume at zero capacityand throughout the entire capacity range. Additional features of theinvention include means for anchoring the wobble plate against rotationwith a universal joint, a means of partially balancing the forces on thepistons and wobble plate to reduce the stress and wear on the movingparts, and an improved capacity control which greatly reduces the lossesof this compressor at partial load compared to all other known means ofcapacity control.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of acompressor constructed in accordance with the principles of the presentinvention with the wobble plate shown in full capacity position;

FIG. 2 is a cross-sectional view taken generally along the plane of line22 of FIG. 1, but with the wobble plate moved to its zero capacityposition so that it rotates in a plane normal to the axis of the driveshaft;

FIG. 3 is a cross-sectional view taken along the plane of line 3-3 ofFIG. 1;

FIG. 4 is a cross-sectional view taken along the plane of line 4-4 ofFIG. 1;

FIG. 5 is a cross-sectional view taken along the plane of line 5-5 ofFIG. I; and

FIG. 6 is a schematic diagram of the capacity control system.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, andparticularly to FIGS. 1 and 2, the compressor of this inventioncomprises a housing 10 having a first open end (left side of FIG. I)closed by a porting plate 12 and its opposite end enclosed by a coverplate 16. Received within housing 10, at the left-hand end thereof (FIG.1), is a cylinder block 18 having a plurality of circumferentiallyspaced cylinders 20 formed therein. Drive shaft 22 has one end supportedwithin a bore 24 in the cylinder block I8 by needle bearings 26 and isheld near its opposite end by bearing assembly 27. This shaft is ofrectangular cross-section for that portion of its length which extendsbetween bearing 26 and the reaction ring 80. Shaft 22 is driven by apulley 28 which is secured to the right-hand end (FIG. 2) of the driveshaft by a key 30 and threaded collar 31. The pulley, at least in anautomotive air conditioning application, would normally be driven fromthe engine through a V-belt. A sea] assembly 32 surrounds the driveshaft and cooperates with the inside surface of cover plate 16 toprovide a fluid tight, rotatable connection.

The pistons 34, which reciprocate in cylinders 20, are driven by awobble plate 36 through connecting rods 38 each having a first ball andsocket fitting 40 connecting them to the pistons and a second ball andsocket fitting 42 connecting them to the wobble plate 36.

The wobble plate 36 is anchored against rotation by means of a universaljoint assembly, designated generally at 46. The universal jointcomprises a yoke 48 having oppositely disposed trunnions 49, 50 receivedin bearings 51, 52 in the wobble plate 36. The yoke 48 is supported forjournalled movement along the other axis by a pair of control pistons53, 54 received in complementary bores 55 and 56in the cylinder block.Each control piston has a pivot 57 receivable in bearings 58 in the yoketo permit the latter to rock back and forth along a horizontal axis.

The drive shaft 22 supports a drive ring 60 having a cylindrical outsidesurface and a pair of ears 62 extending forwardly and connected to eachother by means of a pin 64 which extends through an elongated slot 66 inthe drive shaft. The two inner surfaces of the drive ring and its earsare in sliding engagement with the rectangular portion of drive shaft 22as shown in FIGS. 2 and 3. The wobble plate 36 is supported on the drivering,

by means of a ball bearing assembly 72 having its inner race 74 fittedonto the OD. of the drive ring and its outer race 76 fitted into arecess on the inside surface of the wobble plate 36. This, of course,permits the drive ring 60 to rotate about the drive shaft axis butallows the wobble plate to maintain a non-rotatable position withrespect to said axis. When the drive ring and balance ring are inclinedwith respect to the axis, the wobble plate will undergo a nutatingmotion causing axial reciprocating movement of the pistons 34 driven bythe connecting rods 38.

When the wobble plate is normal to the shaft 22 it is obvious thatconventional balancing techniques can result in excellent rotationalbalance and vibration free operation. When the wobble plate is inclined,however, a much more complex situation exists. To obtain ideal balance,it is necessary that the following three conditions be fulfilled (l) themagnitude and distribution of the masses in the rotating elements of thewobble plate assembly must balance the rotating couple generated by thereciprocating pistons. Fortunately, when this is achieved at one angleof inclination and speed, it then holds for all combinations of speedand stroke; (2) the center of gravity of the rotating elements of thewobble plate assembly must fall on the center of the pin 64; and (3) thecenter of gravity of the nutating elements of the wobble plate assemblymust fall on the center of pin 64.

To achieve conditions (I) and (2), the balance ring support 69, which isattached to the back side of drive ring 60 by screws 70, is made ofaluminum or magnesium to keep the weight as low as possible. The balancering 68 is made ofa heavy material such as iron or steel and fastened tosupport 69 with screws 71. To achieve condition (3), six counterweights73 (only one of which is shown FIG. 2) are attached by screws 75 to thewobble plate 36 at points intermediate the connecting rods.

The balance ring 68 is also connected to a reaction ring 80 by means ofa pair of links 82 attached, by means of pins 83, to ears or bossesextending forwardly on the reaction ring such that the pivot point ofthe balance ring is spaced from the drive shaft axis at a point 85 nearthe upper left-hand portion of the apparatus in the position viewed inFIG. 1. The links 82 are pivotably attached to the balance ring 68 bymeans of pins 87 (see FIG. 3). The pivot point 85 is not rigidly locatedwith respect to the shaft, but is on the end of the two links 82 whichare free to pivot on pins 83 and 87, the former being connected to thebosses 84 on reaction ring 80. The use of links instead of a rigidlocation of pivot point 85 permits the pin 64 to remain at all times onthe centerline of the shaft 22 (i.e. a straight slot) which is importantfor balance considerations.

The major axial loads on the wobble plate 36 are transmitted through thebearing 72, the balance ring support 69, and the balance ring 68 to thelinks 82. The links then transmit the axial forces to the reaction ring80, then through the bearing assembly 27 to the housing 10. The reactionring 80 is pressfitted onto, and driven with, the drive shaft 22.

The shaft 22 rotates and carries with it the reaction ring 80 and thedrive ring 60. All of the driving torque is transmitted to the drivering through the closely fitting flats on the shaft 22, and drive ring60. Both the drive ring 60 and the balance ring support 69 are rigidlyclamped to the inner race 74 of the large ball bearing.

The balance ring support 69 is connected to the balance ring 68 by bolts71, and the balance ring 68 is further connected to the reaction ringthrough the links 82 as outlined previously. The links 82 do not carryany torque loads.

The wobble plate 36 is supported on the outer race 76 of the large ballbearing and is anchored against rotation by the universal joint assembly46 and the control rods 53. Now, when the ball bearing is in an inclinedposition as shown in FIG. Land the inner race 74 is forced to rotate bythe shaft 22 and the drive ring 60, the inner race acts as a cam toimpart to the outer race 76 and the wobble plate 36 a nutating motion.

An important feature of the invention resides in the design to balancethe forces acting on the wobble plate. In the. structure as heretoforedescribed, there would always be a tendency for the wobble plate to beurged to its fully inclined, full stroke position. This is because thereaction forces opposing piston movement in the cylinders at the lowerhalf of the compressor (as viewed in FIG. 1) would be inclined to tiltthe wobble plate around the upper pivot point. To compensate for theseforces it is desirable to pressurize the crankcase with gas in such away that the pressure is applied to the underside of each piston. Inthis regard, it has been found that the pressure maintained inside thehousing 10 should be between 5-10 percent of the difference betweensuction and discharge pressure at all times. For example, when operatingat 200 psig discharge and 30 psig suction, the crankcase pressure shouldbe controlled between 38.5 and 47 psig. In practice, the gas blowing bythe piston sealing rings is enough to pressurize the chamber.

A compression spring is provided in a blind bore 92 extending axiallythrough the drive shaft 22. One end of the spring abuts the blind end ofthe bore and the opposite end engages a cylindrical guide 94 throughwhich pin 64 extends. The spring force thus urges the pin 64 and thedrive ring 60 to which it is connected, to the left (as viewed inFIG. 1) to bias the wobble plate in the upright position. When in thezero stroke position, no gas blow-by will be available so the spring 90will hold the wobble plate in this position until the control calls forincreased stroke. The wobble plate is controlled hydraulically by highpressure oil from the discharge side of oil pump 100, the details ofwhich will be given below. As viewed in FIG. 2, it will be noted thatthere is a hollowed out section 102 of cylinder block 18 whichterminates at one end (the left as viewed in FIG. 2) at the portingplate 12 and at the other end in three cylindrical bores 55, 56,previously described, and 59, which is an extension of the borecontaining bearing 26. A piston assembly 106 is arranged in the chamber108 defined by hollowed out section 102 which assembly includes acontrol rod bridge 110, the control pistons 53, 54, respectivelyslidable in the bores 55, 56, and the centrally located piston 61 inbore 59. O-ring seals 111 surround each of the pistons 53, 54 and 61 toisolate the pressure applied to chamber 108 from the pressure in thecrankcase.

As mentioned above, each of pistons 53, 54 has a pivot 57 journalledinto yoke 48 of the universal joint assembly. Accordingly, the angle atwhich the wobble plate is disposed is controlled by the position ofpiston assembly 106 and the two control pistons 53, 54 which urge theuniversal joint yoke 48 forwardly or rearwardly and thereby control theposition of the wobble plate, the drive ring and the balance ring, allof which move together as a unit about the pivot point.

Refrigerant gas is admitted to and discharged from the gas workingspaces 120 (defined by cylinders 20, porting plate 12 and pistons 34)through head 14, which is secured to the porting plate 12 and cylinderblock 18 by capscrews 15. As best shown in FIG. 4, the head is providedwith a pair of closely spaced partition walls 121, 123 that cooperatewith porting plate 12 to divide the head into an outer suction gasplenum 124 and an interior discharge gas chamber 126. Suction plenum 124and discharge chamber 126 are provided with suction and discharge gasconnections or fittings (not shown) to which the gas lines are connectedto the compressor.

Porting plate 12 is provided with first series of suction gas ports 128through which gas flows from suction plenum 124 to the working spaces120 and a second series of discharge ports 130 through which thecompressed gas flows from working spaces 120 to discharge gas chamber126. The suction ports are closed by suction valves 134 and thedischarge ports are closed on the opposite side of porting plate 12 bydischarge valves 136.

The oil pump 100 is disposed at the right-hand end (HO. 1) of theapparatus. The pump comprises a generally elliptically shaped rotor 202,which is secured to and driven by the drive shaft 22, said rotor beingdisposed in a cylindrical pumping chamber 204 formed by stator 206 andtwo spaced end plates 208 and 210. Engaging the surface of the rotor aretwo opposed vanes 212, 214 which project out of the stator and arespring loaded in a radially inward direction to maintain slidingengagement with the rotor surface. The rotor 202 and pumping chamber 204therefor define a pair of pumping cavities 216, 217 which are connectedon the one hand with the suction side of the oil pumping system throughpassages 220 and 222, and to the discharge side of the pumping systemthrough passages 224 and 226 which are connected through passages (notshown) to line 150. The oil is picked up in the lower portion of thehousing which forms an oil sump 230 and flows through angular passage232 to the suction ports communicating with the pumping cavities. Thepressurized oil is used to regulate the stroke of the compressor bymeans of piston assembly 106 together with a suitable control systemdescribed below. Excess oil can be discharged axially through the centerof the drive shaft to various oilpa ssages com mun Eatiri gw it heii'seaaiements such as, for example, bearings 26 and 27.

A suitable capacity control system is illustrated schematically in H6.6. The crankcase oil sump 230, it will be recalled, is connected bymeans of lines 232 to the inlet side of oil pump 100. The flow of oil isdirected through line 150 to chamber 108 controlling the stroke controlpiston assembly 106. In parallel with the oil pump is a branch line 152which includes a relief valve 154 normally set to open at a pressureabout 30-60 psi above crankcase pressure. In parallel with the reliefvalve 154 is a normally open solenoid valve 156 which connects line 150with line 152 downstream from relief valve 154.

A zero-stroke valve 160, the operation of which will be described inmore detail below, is connected via line 162 so as to be actuated by oilpressure in line 150. Valve 160 controls the flow of gas from thecrankcase through line 164 in response to oil pressure under certainconditions. The main control valve 166 comprises a valve element 168controlling flow between lines 164 and line 170, the latter beingconnected with the suction gas line 172. The zero-stroke valve 160 isoperated by oil pressure in opposition to a spring 174 which in theabsence of oil pressure prevents gas flow from the crankcase to the maincontrol valve through line 164. Control valve 166 is controlled inresponse to the pressure sensed in suction line 172 (which is thevariable that is ultimately controlled) by means of a diaphragm 176sensing suction pressure through lines 170 and 171 in cooperation with aspring 177 biasing the diaphragm in the opposite direction. The positionof element 168 controls flow of gas from the crankcase through lines164, the control valve body and line 170.

OPERATION The operation will be described in the context of varying thecapacity from full stroke (with the wobble plate shown in the positionof H0. 1) to the no stroke position (with the wobble plate as shown inFIG. 2). The drive from the V-belt (not shown) is applied to pulley 28which through the connection with drive shaft 22 causes the same torotate. The torque is thus transmitted to drive ring 60 by theengagement of the flats on the drive shaft with the correspondingsurfaces on the drive ring 60. As the drive ring rotates, it causes thewobble plate to follow a nutating path driving the pistons 34 throughconnecting rods 38. The gas admitted to the working spaces throughsuction valves 134 (see FIG. 4) on the opposite side of porting plate12. On the discharge stroke of each cylinder, the gas is forced througha discharge valve 136 into discharge chamber 126.

As long as the full capacity of the compressor is required (i.e. thesuction pressure in line 172 is above the control pressure ofapproximately 30 P510) the gas pressure loads on the pistons 34 hold thewobble plate assembly as its maximum angle of inclination. During thecompression stroke there is leakage of high pressure gas past thepistons (referred to later as blow-by) into the crankcase cavity 151which is vented to suction.

The control shown schematically in FIG. 6, is based on the regulation ofthe escape of blow-by gas from the crankcase to suction which raises orlowers the crankcase pressure in response to a signal from the suctionpressure resulting in a drecrease or increase in the compressordisplacement as follows:

When the air conditioning system is operating, the normally opensolenoid valve 156 is energized and held in the closed position. Thepump 100 draws oil from the sump 230 and elevates it to the pressure setby the relief valve 154. This pressure is communicated through lines tochamber 108 and piston assembly 106 to urge the compressor into stroke,and through 150 and 162 to the annular piston area of element 153 in thezero-stroke valve 160. Element 153 is held up ward against a spring 174and opens the line 164 venting the crankcase through lines 164, controlvalve 166, and line to suction line 172. When the suction pressure inline 172 gets down to the control pressure (sensed on the backside ofdiaphragm 176 through lines 170 and 171) the spring 177 moves thediaphragm 176 and valve 168 downward thereby restricting thecommunication between the crankcase 151 and the suction line 172. Sincethe leakage of gas past the pistons into the crankcase continues, thisrestriction in the body of valve 166 causes an increase in pressure inthe crankcase 151 of the compressor. This pressure acting on theunderside of the piston 34 results in a decrease in angularity of thewobble plate mechanism and consequently a reduction in the piston strokeand the displacement of the compressor. Similarly, it can be seen thatwhen the compressor is at less than the minimum stroke, an increase inpressure in line 172 will cause the valve element 168 to move upwardreducing the restriction between the crankcase 151 and suction line 172.This increases the flow rate of blow-by gases out of the crankcase andreduces the crankcase pressure allowing the compressor displacement toincrease and lower the suction pressure in line 172. Experimentalmeasurements taken from a compressor built an operating in the describedmanner show that the crankcase pressure normally falls between and ofthe difference between suction and discharge pressure.

When the system is turned off, the normally open solenoid valve 156 isde-energized so that it opens and permits a free circulation of oil backto the sump 230. This reduces the oil pressure behind the pistonassembly 106 and beneath the annular piston of the movable element 153in the zero-stroke valve 160. Spring 174 forces the movable element 153downward to close the passage 164. The trapped'blow-by gas rapidly mixesthe crankcase pressure sufficiently to return the wobble plate to thezero-stroke condition.

While the invention has been described in connection with a certainspecific embodiment thereof, this is by way of illustration and not byway of limitation, and the scope of the appended claims should beconstrued as broadly as the prior art will permit.

What is claimed is:

l. A compressor comprising means defining a plurality of gas workingspaces each having a piston cooperating with suction and discharge portsto compress a fluid therein; a drive shaft; a cam mechanism driven bysaid drive shaft; a wobble plate driven by said cam mechanism in anutating path about the drive shaft axis; means operably connectedbetween said wobble plate and the individual pistons to impartreciprocating drive to said pistons, the length of stroke being afunction of the angle at which said wobble plate is supported relativeto the drive axis; means for pivoting said wobble plate at a pointspaced from said drive axis so as to permit said wobble plate to beinclined relative to a plane normal to said drive shaft axis, so thatwhen the wobble plate is disposed in said normal plane, the pistons arelocated at top-dead-center within said gas working spaces; means forproducing a substantially constant force biasing said wobble plate awayfrom said normal plane to increase the length of stroke; a fluid tighthousing'enclosing said pistons and confining the fluid bypassing saidpistons from said gas working spaces such that the fluid pressure insaid housing is applied to the underside of each said piston to producea force urging said wobble plate toward said normal plane; and pressurecontrol means adapted to control the pressure in said housing inresponse to capacity requirements of said compressor.

2. A compressor as defined in claim 1 wherein the pressure in saidhousing is maintained above suction pressure by an amount equal to 5-10percent of the difference between discharge and suction pressure.

' 3. A compressor comprising means defining a plurality of gas workingspaced arranged in spaced relation along generally parallel axes, eachgas working space being provided by a stationary cylinder, and a piston5 cooperating with suction and discharge ports to compress a gaseousfluid therein; a wobble plate driven in a nutating path by a rotatingcam member; said wobble plate being supported at a point spaced from itscentral axis and being pivotable from a first plane wherein said wobbleplate lies substantially normal to the axis of said gas working spacesto a second plane wherein said wobble plate is substantially inclinedrelative to said first plane; means for connecting said wobble plate tosaid individual pistons to impart reciprocating motion to said pistons,the length of stroke being a function of the angle at which said wobbleplate is supported relative to said working space axes; means formaintaining a substantially constant force urging said wobble plate in adirection toward said second position; housing means providing asubstantially closed crankcase, the pressure in said crankcase beingapplied to the underside of each said piston, the gaseous fluidby-passing said pistons into said crankcase developing a pressure,subject to modulating control, for opposing the force tending to urgesaid wobble plate from said first plane to said second plane.

4. A method for controlling the stroke length of a variabledisplacement, wobble plate compressor of the type including a pluralityof gas working spaces including cylinders and pistons arranged alongparallel axes and a wobble plate driving said pistons and being pivotedat a point spaced from its central axis and movable from a firstposition wherein the pistons are at their substantially top-dead-centerpositions and said wobble plate lies in a plane substantially normal tosaid gas working space axes and a second position wherein said wobbleplate lies in a plane which is inclined from said normal plane and saidpistons reach full stroke comprising the steps of providing relativelyconstant force on said wobble plate urging it in a direction toward saidsecond position, and providing a modulated pressure on the underside ofeach said piston, urging said wobble plate toward said first position,by controlling the pressure applied thereto as generated from gasbypassing said pistons in the cylinder.

5. A compressor comprising a housing; means defining a plurality of gasworking spaces each having a movable piston cooperating with suction anddischarge ports to compress a fluid therein; a drive shaft; a cammechanism driven by said drive shaft; a wobble plate driven by said cammechanism, said cam mechanism including a drive ring rotatably driven bysaid drive shaft and adapted to pivot around a point along the driveaxis such that the drive ring may be inclined relative thereto, and abearing assembly interconnecting said wobble plate and said drive ringsuch that the wobble plate follows a nutating motion when so inclined; aplurality of connecting rods operably connecting said wobble plate andthe individual pistons; a universal joint assembly including a yokeconnected to said wobble plate at two points along a first axis; meansfor connecting said yoke to an axially movable member along a secondaxis disposed at right angles to said first axis whereby said wobbleplate is free to pivot about both of said axes to permit nutatingmovement relative to said drive axis; and an annular element carried bysaid drive ring providing a substantial mass radially out- 10 drive ringpivot point.

7. A compressor as defined in claim 6 including a reaction ringrotatable with said drive ring and said drive shaft; and means forpivotally supporting said balance ring from said reaction ring, wherebythe reaction forces are transmitted through the reaction ring to thehousing.

1. A compressor comprising means defining a plurality of gas workingspaces each having a piston cooperating with suction and discharge portsto compress a fluid therein; a drive shaft; a cam mechanism driven bysaid drive shaft; a wobble plate driven by said cam mechanism in anutating path about the drive shaft axis; means operably connectedbetween said wobble plate and the individual pistons to impartreciprocating drive to said pistons, the length of stroke being afunction of the angle at which said wobble plate is supported relativeto the drive axis; means for pivoting said wobble plate at a pointspaced from said drive axis so as to permit said wobble plate to beinclined relative to a plane normal to said drive shaft axis, so thatwhen the wobble plate is disposed in said normal plane, the pistons arelocated at top-dead-center within said gas working spaces; means forproducing a substantially constant force biasing said wobble plate awayfrom said normal plane to increase the length of stroke; a fluid tighthousing enclosing said pistons and confining the fluid bypassing saidpistons from said gas working spaces such that the fluid pressure insaid housing is applied to the underside of each said piston to producea force urging said wobble plate toward said normal plane; and pressurecontrol means adapted to control the pressure in said housing inresponse to capacity requirements of said compressor.
 2. A compressor asdefined in claim 1 wherein the pressure in said housing is maintained inthe range of 5-10 percent of the difference between discharge andsuction pressure.
 3. A compressor comprising means defining a pluralityof gas working spaces arranged in spaced relation along generallyparallel axes, each gas working space being provided by a stationarycylinder, and a piston cooperating with suction and discharge ports tocompress a gaseous fluid therein; a wobble plate driven in a nutatingpath by a rotating cam member; said wobble plate being supported at apoint spaced from its central axis and being pivotable from a firstplane wherein said wobble plate lies substantially normal to the axis ofsaid gas working spaces to a second plane wherein said wobble plate issubstantially inclined relative to said first plane; means forconnecting said wobble plate to said individual pistons to impartreciprocating motion to said pistons, the length of stroke being afunction of the angle at which said wobble plate is supported relativeto said working space axes; means for maintaining a substantiallyconstant force urging said wobble plate in a direction toward saidsecond position; housing means providing a substantially closedcrankcase, the pressure in said crankcase being applied to the undersideof each said piston, the gaseous fluid by-passing said pistons into saidcrankcase developing a pressure, subject to modulating control, foropposing the force tending to urge said wobble plate from said firstplane to said second plane.
 4. A method for controlling the strokelength of a variable displacement, wobble plate compressor of the typeincluding a plurality of gas working spaces including cylinders andpistons arranged along parallel axes and a wobble plate driving saidpistons and being pivoted at a point spaced from its central axis andmovable from a first position wherein the pistons are at theirsubstantially top-dead-center positions and said wobble plate lies in aplane substantially normal to said gas working space axes and a secondposition wherein said wobble plate lies in a plane which is inclinedfrom said normal plane and said pistons reach full stroke comprising thesteps of providing relatively constant force on said wobble plate urgingit in a direction toward said second position, and providing a modulatedpressure on the underside of each said piston, urging said wobble platetoward said first position, by controlling the pressure applied theretoas generated from gas bypassing said pistons in the cylinder.
 5. Acompressor comprising a housing; means defining a plurality of gasworking spaces each having a movable piston cooperating with suction anddischarge ports to compress a fluid therein; a drive shaft; a cammechanism driven by said drive shaft; A wobble plate driven by said cammechanism, said cam mechanism including a drive ring rotatably driven bysaid drive shaft and adapted to pivot around a point along the driveaxis such that the drive ring may be inclined relative thereto, and abearing assembly interconnecting said wobble plate and said drive ringsuch that the wobble plate follows a nutating motion when so inclined; aplurality of connecting rods operably connecting said wobble plate andthe individual pistons; a universal joint assembly including a yokeconnected to said wobble plate at two points along a first axis; meansfor connecting said yoke to an axially movable member along a secondaxis disposed at right angles to said first axis whereby said wobbleplate is free to pivot about both of said axes to permit nutatingmovement relative to said drive axis; and an annular element carried bysaid drive ring providing a substantial mass radially outwardly of saiddrive shaft axis and between said drive ring pivot point and said gasworking spaces to balance the wobble plate when the same is inclinedrelative to the drive axis and maintain the center of gravity of therotating elements in the drive ring assembly close to said drive ringpivot point.
 6. Apparatus as defined in claim 5 including counterweightsconnected to said wobble plate to maintain the center of gravity of saidnutating elements close to said drive ring pivot point.
 7. A compressoras defined in claim 6 including a reaction ring rotatable with saiddrive ring and said drive shaft; and means for pivotally supporting saidbalance ring from said reaction ring, whereby the reaction forces aretransmitted through the reaction ring to the housing.